James Dredge's lightweight suspension bridge over the River Oich, near the point where the river leaves Loch Oich in Scotland's Great Glen, once carried the main road. Now refurbished and maintained by Historic Scotland, it is used as a footbridge. Loch Oich is the highest part of the Caledonian Canal, which is adjacent to the river at this spot.

James Dredge (1794-1863) was a brewer from Bath in Somerset. He took up engineering in the 1830s — perhaps as a way of developing better transport routes for the beer. He constructed some 50 bridges of this type between 1836 and 1854, of which the Bridge of Oich is now the best surviving example in Scotland. According to an 1843 advertisement for Dredge's Patent Iron Bridges, the largest of his Scottish bridges crossed the River Leven and was 89m long and some 6m wide.

Though appearing to be suspension bridges, Dredge's structures are actually double cantilevers, with each pier supporting the deck load either side of it by means of wrought iron chains. He based his chain layout on a tapering principle, and patented his design on 22nd July 1836. It enabled significant savings in ironwork and construction time.

The Bridge of Oich has a span of 47.4m and its chain runs are 5.2m apart. The chains consist of a series of wrought iron rods that vary in length from 1.8m to 2.3m, with a 22mm nominal diameter. They pass over two 5.5m high masonry towers, meeting at the bridge's centre where they are anchored to the deck.

At the towers, the chains are made up of 12 rods in parallel. The number of rods reduces progressively towards mid span, and similarly on the landward side of each tower, where they are anchored into the ground. Trussed wrought iron transoms support the timber decking transversely.

By progressively reducing the cross-section of the chains towards mid span, by omitting a pair of rods at each suspender position, Dredge produced a light but strong bridge. The rigid rods, rather than the cables of a traditional suspension bridge, also helped reduce sway in the deck.

The bridge was almost certainly erected without scaffolding, in "barely two months of workable time. The whole of the ironwork about twenty tons was forged and fitted upon the spot, with the exception of a small portion of metal castings, and the Highlanders in the locality of the bridge, with the exception of three smiths and a few Aberdeen masons, did the whole of the work; and the bridge was asphalted, painted, gravelled and completed, at less cost than that of a rude timber bridge".

The Bridge of Oich carried road traffic until 1932, when it was bypassed by the nearby three-span concrete bridge carrying the A82. After that, it wasn't maintained very well and eventually closed. It has been a Category A listed building since October 1971, but by the 1990s it was one of only seven Dredge bridges still standing.

In 1995-7, the Bridge of Oich was strengthened and refurbished by Halcrow Crouch, at a cost of £280,000. Its lightweight construction has meant that its live loading is limited to just 1kN per sq m. The bridge re-opened to pedestrians in September 1997.

Main contractor (1997): Morrison Construction

Research: ECPK

bibliography

"Analysis of James Dredge's Victoria Bridge, Bath" by R.A. Griffiths, in Proceedings of Bridge Engineering 2 Conference 2009, University of Bath, Bath, April 2009